Next to pots, furnaces are most important for the success of a glass
manufactory. Long ago it was seen that the old English plan was
defective. They consumed coal at an extravagant rate, though this was
not a serious drawback in England, because the furnaces were located
near coal-mines, and run with a quality called slack, not otherwise
merchantable. English furnaces were constructed with reference to
durability, usually eight feet in diameter at the interior base, and
six feet clear at the crown. This rule was followed in this country
until 1840. The writer, having occasion to build an extra furnace,
adopted the novel plan of one fourteen feet diameter at the base in the
clear and only five feet at the crown, braced by binders, with
cross-ties to prevent lateral expansion, which was a success.
A furnace on the old plan consumed 2575 bushels of coal weekly, and
refined only 38,000 pounds of raw material. The new refined 35,000
pounds, with a consumption of only 2000 bushels of coal. Since then a
further decrease in consumption of coal has been produced by the use of
the Delano patent, which feeds the furnace by forcing up the coal at
the bottom of the burning mass, thus consuming the entire smoke, and
obviating the necessity of wheeling coal on the glass-house floor and
impeding the workmen. It also does away with all danger to the pots in
feeding the fires. Besides these great advantages, it distributes a
regular and uniform heat to each pot, causing the pots to last much
longer, and fusing the metal better,--important items to mixers.
From three to five tons of fuel is the weekly saving in a first-class
furnace.
It is of vital importance to obtain pots that will last a reasonable
time. Clays of the finest quality are essential. Each piece must be
freed from any foreign matter, particularly sulphate of iron, which
often occurs. The burnt and raw clay should be well mixed, wet, and
frequently kneaded, or trod over by the naked feet. Tenacity must be
secured, sufficient that a roll twelve to eighteen inches long can be
suspended, and hold firmly together by its own adhesiveness. The next
point is to make the pots free from air blisters, all portions being
compact; then to dry them thoroughly, which requires great care on
account of the inequality of the different parts. Pot-makers are not
agreed as to the value of different clays, and the use and proportion
of raw to burnt shells. Some use sixteen parts raw to eleven burnt,
some fifty-five raw to forty-five burnt, some equal proportions of
each.
Manufacturers have mainly depended upon imported clays, but the Western
glass-makers have used Missouri clay with success. In the east it has
not yet come into general use. Of the imported, that from Stowbridge is
considered best. Garnkerk is a strong clay, and, if well selected, will
rival any other. The analyses are for